274 DIPTERA. 



although hving in situations that seem to demand their 

 development, shows that they must have inherited this 

 peculiarity from an ancestral form whose larva had 

 lost them. This comparative inflexibility of the larval 

 stage is sufficient of itself to show that there is now a 

 wide gap between the existing Diptera and all other 

 orders of insects, and that this chasm is not closed by 

 the resemblances of the parts in the adult to those of 

 the Lepidoptera or isolated forms in other orders. 



There are in this order also marks of extreme spe- 

 cialization in the mouth parts of the adult, which are, 

 as a rule, modified for the office of sucking. The 

 abdomen has not the flexibility of the pedunculated 

 abdomen of the Hymenoptera Aculeata, no stinging- 

 apparatus being present, but it is, nevertheless, nar- 

 rowly pedunculated in some forms. The aspect of 

 the highly complicated and concentrated thorax ac- 

 companies the reduction of the wings to one func- 

 tional pair. This last characteristic and the tendency 

 to reduce the useless pair of wings is carried to an 

 extreme throughout this order, and can thus be 

 compared as a whole with such isolated speciahzed 

 types in other orders as the Coccidae among Hemip- 

 tera, and the Stylopidae among Coleoptera. 



The tendency to the enlargement of one pair of 

 wings, like the tendency to the enlargement of certain 

 pairs of thoracic legs and the reduction of other pairs, 

 or a change in their structure and function so that the 

 insect makes a departure from the conventional nor- 

 mal type of four equal membranous wings and six 

 equal-jointed legs, is everywhere an index of speciali- 

 zation. 



